In a facility of the type to which the present invention relates, mine-run material is initially fed to a crushing plant that reduces it to particles of sizes suitable for feed to a grinding mill, and the grinding mill further reduces the material to a specified final particle size. Typically, the particle size of material fed to the grinding mill will be such that all of the material can pass through a 1/2 inch (12.7 mm.) mesh screen. It is recognized that some substantially larger particles or chunks are fed into an autogenous mill, but they are a minor portion of the input and can be considered as grinding agents rather than as material to be ground.
Disregarding such grinding agents, it is generally true that particles fed to a grinding mill should be no larger than a specified size but can be smaller than that size. In fact, it is desirable that the material be reduced as much as possible in the crushing plant because a crushing plant utilizes power more efficiently than a grinding mill. Other things being equal, where particles of material are of such size that they are acceptable for feed to a grinding mill, but could be further reduced in a crushing plant, the crushing plant will expend about half as much energy as the grinding mill in reducing the size of such particles by a given amount. However, any reduction in particle size of crushing plant product tends to be obtained at the expense of rate of output of the crushing plant because, other things being equal, the smaller the particle size of crushed product that it delivers, the less will be the quantity of such product that the plant produces in a given time.
Energy economy in the grinding mill component of a comminuting facility is manifested in faster grinding of the smaller particles, and hence in a higher rate of production for a given energy expenditure. Furthermore, this higher rate of production constitutes a more profitable utilization of the capital investment represented by the mill and is accompanied by decreased wear and tear on equipment and in lower labor and maintenance costs per ton of final product. In a sense, therefore, any reduction in the size of the coarsest particles fed to a grinding mill can result in a compounded economic gain. But of course this gain cannot be fully realized if the mill must at times stand idle because its production rate has exceeded that of a crushing plant which feeds it.
Considering a crushing plant and an associated grinding mill as a complete facility, it follows from the superior energy efficiency of the crushing plant that the greater the amount of energy expended by it in actual crushing of material, the less is the total amount of energy consumed by the complete facility in processing a given quantity of mine-run material to final product. However, energy efficiency is only one factor in the economical operation of the facility; for the most profitable utilization of the invested capital, the production rate of the crushing plant should be matched to that of the grinding mill, so that neither is idle when it could be working.
My copending application discloses a method of operating a crushing plant to greatest advantage, but it treats the crushing plant as an independently operating entity which functions without regard to possible variations in the amount of feed material required by an associated grinding mill. According to that method, the rate of feed of raw material to the crushing plant is so controlled as to ensure that the plant will operate through the whole of each of its working periods to produce neither substantially more nor substantially less than a predetermined quota, and the operation of the crushing plant is further so controlled that at all times during the period it consumes the maximum amount of power available to it. With such operation of a crushing plant, its output will have optimum economic value because it will be in the smallest particle size attainable under the constraint imposed by the production quota and by existing physical conditions including the nature of the material to be crushed.
There are situations in which the daily quota for a crushing plant is established arbitrarily, as by fixed contractual obligations. In such cases the problem of operating the crushing plant to obtain an economically optimum product is solved by the invention of the copending application, now U.S. Pat. No. 4,179,074.
But there are a good many cases in which the determination of a proper quota for a crushing plant is in itself a problem. By way of example, if a grinding mill fed with the output of a crushing plant has a capacity such that it can process 75,000 tons of certain crushed product during each working day, and the program for the crushing plant yields 100,000 tons of crushed product per working day, there will be an unbalance between the crushing and the milling operations that will require early shutdown of the crushing plant each day. The facility that comprises the crushing plant and the grinding mill, considered as a whole, will not be operating to maximum advantage under these conditions, because the total available energy will not have been applied to the final product with optimum efficiency, the capital invested in the crushing plant will have yielded no return during its idle time, and substantial storage space will have had to be provided to accommodate the difference in production rates between the two operations. Furthermore, because all of the energy available to the crushing plant will not have been applied to its output, that output will be a relatively coarse material that will subject the components of the grinding mill to undue wear.
On the other hand, decreasing the quota of the crushing plant to the assumed 75,000 ton capacity of the grinding mill, and operating the crushing plant to require the whole of its working day for processing that tonnage would result in a substantial increase in the capacity of the grinding mill, owing to the smaller particle size of the crushed product fed to it from the crushing plant. In that case the crushing plant would not be supplying enough feed material for operation of the grinding mill at its full capacity, and again the facility as a whole would not be operating for optimum return on invested capital.
Thus the substantial capital investment in a facility comprising a crushing plant and a grinding mill is employed to full advantage only when each of those operations is working through the whole of the time in which it can normally be in operation. On the other hand, optimum energy efficiency is obtained with the facility if the crushing plant always consumes the maximum amount of energy available to it and produces exactly the quantity of crushed product that will keep the grinding mill operating full time.
From that analysis it is obvious that efficient operation of a comminuting facility requires maintenance of a substantial balance between the daily output of its crushing plant and the daily feed requirement of its grinding mill. What is not at all obvious--as is apparent from the simplified example given above--is how such balanced operation of a comminuting facility can be achieved along with optimum energy efficiency. Briefly stated, the problem is that the crushing plant cannot be operated to meet some fixed quota set by the grinding mill because the operation of the crushing plant controls the capacity of the grinding mill, which in turn tends to control how the crushing plant shall be operated.
There does not seem to be any mathematical model that could be reliably used for maintaining a balance between output of a grinding mill and output of an associated crushing plant, owing to the several unascertainable variables affecting that balance.
One factor affecting the problem is that the two types of operations tend to have different working cycles. Typically, a grinding mill can operate, on average, about 23 hours out of 24 and needs the equivalent of about one hour a day for maintenance, whereas a crushing plant operates an average of about 20 hours out of 24 and must be idle for maintenance during the remaining time. Although the availability of both the crushing plant and the grinding mill can be known in terms of daily averages over an extended period, particular episodes of down-time do not necessarily occur at regular intervals in either case.
A more difficult variable to deal with is presented by the material fed into the facility. To obtain maximum production for energy expended, both the grinding mill and the crushing plant must be so operated that each draws the full amount of power available to it all during the time that it is in operation, but the quantity of output obtained for a given energy expenditure by either operation depends upon the comminution properties of the material being processed and the various proportions of particle sizes in the mine-run raw material being fed to the facility. These are virtually unpredictable. If a balance between crushing plant and grinding mill outputs occurs by chance under one set of conditions, any change in the coarseness or crushability of the mine-run material fed to the facility will destroy that balance.
The general object of the present invention is to provide a method and apparatus for operating a comminuting facility comprising a crushing plant feeding into a grinding mill whereby the output of the crushing plant is balanced to the feed requirements of the grinding mill while each is operating full time and drawing all of the power available to it, thus enabling the capacities of the facility to be fully utilized for production of final product at the lowest possible cost per unit quantity and at the highest attainable rate.
Another and very important general object of the invention is to achieve substantial conservation of energy in the comminuting of minerals by providing for operation of a comminuting facility of the character described in a manner that will assure the most efficient utilization of the energy available to the facility and will consequently enable its final product to be produced at the lowest attainable energy cost per unit quantity.
It is also an object of this invention to provide a method of operating a comminuting facility comprising a crushing plant feeding into a grinding mill, wherein the crushing plant is operated in accordance with the method of the above-identified copending application, to afford maximum efficiency in the utilization of the power available to it, and whereby the operation of the crushing plant is nevertheless coordinated with operation of the grinding mill to afford optimum utilization of the facility as a whole and maximum efficiency in its overall operation.
A further object of the invention is to provide a method and apparatus for maintaining a constant substantial balance between the outputs of a crushing plant and of a grinding mill into which the crushing plant feeds, taking account of all of the variables and unknown quantities that influence the output of each while enabling both to be operated at optimum efficiency.
An additional important object of this invention is to make possible the efficient and satisfactory operation of a grinding mill that comprises on or more very large machines, and to overcome serious disadvantages heretofore encountered because of scale effect with grinding mill machines larger than a conventional size range.
It is known that as the size of grinding mill machines increases above the conventional size range, the capital cost of such very large machines does not increase in direct proportion to production capacity. With increasing size for larger machines, there is also a decrease in maintenance costs and non-energy operating costs as measured on the basis of tons per hour of final product, as well as a desirable saving in floor space. A very large machine, in and of itself, does not necessarily consume any more energy than a small one in effecting a given reduction of particle size of a given volume of a particular material. However, the volume of a grinding mill drum increases with the square of its diameter, whereas the power required to drive the machine increases as an exponential function of drum diameter that is substantially higher than 2. This means that the retention time per unit of energy input decreases with increasing drum size. As a result of this scale effect, the product that issues from a very large machine tends to be substantially coarser than that from a machine of conventional size; and therefore a grinding mill comprising a very large machine must also have relatively costly and elaborate classification and recirculating equipment whereby particles that are of larger than a desired final product size are separated from the rest of the output of the machine and sent back for further grinding. Such recirculation of course consumes energy, and the larger the recirculating load, the greater is the amount of energy consumed in recirculation, so that total energy consumption of a grinding mill comprising one or more very large machines may be greater than that of a mill comprising smaller conventional machines producing an equivalent output of like material reduced to the same particle size.
Aside from these energy cost considerations, it was found in some prior experiences with grinding mills comprising such larger machines that economically feasible operation was possible only upon condition that a final product be accepted that was coarser and less valuable than was desired. Owing in part to inherent deficiencies in classification equipment, an attempt to obtain a final product of the desired fineness would raise the rates of recirculation to such high levels that the quantity of recirculated material controlled the infeed of fresh material and reduced productivity to a point at which the operation as a whole became uneconomical.
Thus, another object of this invention as it relates to a comminuting facility that comprises a grinding mill equipped with very large machines is to secure the benefits of such machines without incurring the disadvantages which heretofore attended their use.
More specifically in this connection, it is an object of this invention to provide for the operation of such a facility in a manner that minimizes recirculating loads in the grinding mill so that the mill can be maintained in stable operation while producing a final product of desirably fine particle size with a total energy consumption for the mill as a whole that compares favorably with energy consumption by a grinding mill equipped with machines of conventionally smaller size and having an equivalent output of like material.
In connection with this last stated object of the invention, it is more particularly an object to provide for such operation of a facility as a whole that comprises a crushing plant and a grinding mill equipped with very large machines that at least a major portion of the material fed to the grinding mill from the crushing plant will be of small enough particle size to be capable of reduction to a desired final product size in a single pass through a grinding mill machine, thus in some cases eliminating the need for recirculating loads in the grinding mill that consume essentially unproductive energy, or, if recirculation is needed, reducing recirculating loads to a level low enough to avoid operating problems.
It is also an object of this invention to provide a simple method whereby a comminuting facility of the character described can be operated to achieve the above-stated objectives, and simple and inexpensive apparatus, comprising generally conventional components, by which the method can be implemented to provide for substantially automatic control of the operation of the facility.